The present invention relates to multiple beam lasertrons.
Electronic tubes called "lasertrons" are known from articles and from the U.S. Pat. No. 4,313,072.
In these tubes a photocathode is illuminated by a laser beam whose wave length is chosen as a function of the output work of the material from which the phtocathode is formed. Thus, a laser beam pulsed at the frequency F tears packets of electrons from the photocathode at the same frequency F. These packets of electrons are then accelerated in an electrostatic electric field and thus gain in kinetic energy. They then pass through a cavity resonating at frequency F and their kinetic energy is transformed into electromagnetic energy at frequency F. The energy is taken from the cavity by coupling it to an external user circuit.
In FIGS. 1 and 2, two embodiments of lasertrons of the prior art have been shown schematically in longitudinal section.
In these FIGS., the references 1, 2 and 3 designate respectively the photocathode, the laser beam and the electron beam.
In the embodiment shown in FIG. 1, the photocathode 1 is illuminated obliquely by the laser beam 2 and the electron beam 3 propagates along the longitudinal axis XX' of the tube.
In the embodiment of FIG. 2, the laser beam 2 and the electron beam 3 propagate along the longitudinal axis XX' of the tube, but in the opposite direction.
The laser beam 2 is therefore normal to the emissive surface of the photocathode.
The electron beam 3 is accelerated by the electrostatic electric field created by an anode 4, then penetrates into a cavity 5 resonating at frequency F. A collector 6 then receives the electron beam. The electromagnetic energy is taken at frequency F from cavity 5 by coupling it to an external user circuit by a guide wave 7, associated with a window 8, as shown in FIG. 1, or by a loop 9, as shown in FIG. 2.
The advantage of lasertrons is that they are very compact tubes. In lasertrons, electron packets are torn from the photocathode at frequency F. Whereas in tubes such a klystrons, several cavities must be used for distributing the electrons of an initially continuous beam in packets.
The problem which arises with lasertrons is that they are limited in frequency and in power.
Thus, for example, in order to obtain high powers, a large current must be extracted, which requires a cathode with a large surface and the passage of a considerable beam through the cavity. The dimensions of the cavity must then be sufficient to allow the passage of this beam, which limits the operating frequency. In addition, the use of a large sized cavity results in poor coupling between the beam and the cavity, which leads to poor efficiency.
The embodiments of lasertrons which are shown in FIGS. 1 and 2 have the following drawbacks:
in the embodiment shown in FIG. 1, the photocathode is illuminated obliquely. The result is, on the one hand, poor light efficiency of the photocathode and, on the other hand, a laser beam illumination device which must be made as compact as possible for housing it in the vicinity of high voltage parts;
in the embodiment shown in FIG. 2, the laser beam and the electron beam follow the same path. Consequently, the surface of the photocathode which receives the laser beam is limited by the diameter D of the sliding tube of cavity 5 which allows these beams to pass. Furthermore, the laser beam illumination device is subjected to the bombardment of the electron beam.